Fluidic architecture for metal-halogen flow battery

ABSTRACT

A metal-halogen flow battery system includes a stack of flow cells, an electrolyte reservoir and one or more of a concentrated halogen return line fluidly connecting the stack to the reservoir, a venturi, a mixer, a concentrated halogen pump, or a concentrated halogen line heater.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

The present application is a divisional of U.S. application Ser. No.14/230,813, filed Mar. 31, 2014, which is a continuation-in-part of PCTapplication number PCT/US2013/031952 titled “Fluidic Architecture forMetal-Halogen Flow Battery” filed Mar. 15, 2013, which is a continuationof U.S. patent application Ser. No. 13/630,572 entitled “FluidicArchitecture for Metal-Halogen Flow Battery” filed Sep. 28, 2012, whichclaims the benefit of priority to U.S. Provisional Patent ApplicationNo. 61/621,257, entitled “Fluidic Architecture for Metal-Halogen FlowBattery” filed on Apr. 6, 2012. The entire contents of all threeapplications are incorporated herein by reference.

FIELD

The present invention is directed to electrochemical systems and methodsof using same.

BACKGROUND

The development of renewable energy sources has revitalized the need forlarge-scale batteries for off-peak energy storage. The requirements forsuch an application differ from those of other types of rechargeablebatteries such as lead-acid batteries. Batteries for off-peak energystorage in the power grid generally are required to be of low capitalcost, long cycle life, high efficiency, and low maintenance.

One type of electrochemical energy system suitable for such an energystorage is a so-called “flow battery” which uses a halogen component forreduction at a normally positive electrode in discharge mode, and anoxidizable metal adapted to become oxidized at a normally negativeelectrode during the normal operation of the electrochemical system. Anaqueous metal halide electrolyte is used to replenish the supply ofhalogen component as it becomes reduced at the positive electrode. Theelectrolyte is circulated between the electrode area and a reservoirarea. One example of such a system uses zinc as the metal and chlorineas the halogen.

Such electrochemical energy systems are described in, for example, U.S.Pat. No. 3,713,888, 3,993,502, 4,001,036, 4,072,540, 4,146,680, and4,414,292, and in EPRI Report EM-1051 (Parts 1-3) dated April 1979,published by the Electric Power Research Institute, the disclosures ofwhich are hereby incorporated by reference in their entirety.

SUMMARY

An embodiment relates to metal-halogen flow battery system includes astack of flow cells, an electrolyte reservoir and one or more of aconcentrated halogen return line fluidly connecting the stack to thereservoir, a venturi, a mixer, a concentrated halogen pump, or aconcentrated halogen line heater.

Another embodiment relates to a method of using the flow battery systemdescribed above.

Another embodiment relates to a method of separating the stack outletstream into a concentrated halogen from the aqueous metal halideelectrolyte outside of the stack and providing the concentrated halogeninto a lower portion of the reservoir via a concentrated halogen returnline.

Another embodiment relates to a method of operating a flow batterycomprising a stack of flow cells in which each flow cell in the stackincludes a fluid permeable electrode, a fluid impermeable electrode, anda reaction zone between the permeable and impermeable electrodes. Themethod includes the following steps.

(a) In charge mode, plating a metal layer on the impermeable electrodeof each cell in the reaction zone by: (i) flowing a metal halideelectrolyte from a reservoir through an inlet conduit to the reactionzone of each flow cell in the stack in a first direction, such that amajority of the metal halide electrolyte enters the reaction zone fromthe inlet conduit without first flowing through the permeable electrodein the flow cell or through a flow channel located between adjacent flowcell electrodes in the stack; and (ii) flowing the metal halideelectrolyte from the reaction zone of each flow cell in the stackthrough a first outlet conduit to the reservoir, such that the majorityof the metal halide electrolyte does not pass through the permeableelectrode in each flow cell before reaching the first outlet conduit;and

(b) In discharge mode, de-plating the metal layer on the impermeableelectrode of each cell in the reaction zone by: (i) flowing a mixture ofthe metal halide electrolyte and a concentrated halogen reactant fromthe reservoir through the inlet conduit to the reaction zone of eachflow cell in the stack in the first direction, such that a majority ofthe mixture enters the reaction zone from the inlet conduit withoutfirst flowing through the permeable electrode in the flow cells orthrough the flow channel located between adjacent flow cell electrodesin the stack; and (ii) flowing the mixture from the reaction zone ofeach flow cell in the stack through a second outlet conduit to thereservoir, such that a majority of the mixture passes through thepermeable electrode in each flow cell before reaching the second outletconduit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a side cross sectional view of an embodiment of theelectrochemical system with a sealed container containing a stack ofelectrochemical cells.

FIG. 2A illustrates a schematic side cross sectional view of flow pathsin the embodiment electrochemical system.

FIGS. 2B and 2C illustrate schematic side cross sectional views of flowpaths in the flow battery cells of the system of FIG. 2A.

FIG. 3A is a plan view of an upper side of a cell frame for holding thehorizontally positioned cells illustrated in FIGS. 2A-2C.

FIG. 3B is a plan view of a lower side of the cell frame illustrated inFIG. 3A.

FIGS. 3C and 3D are respective three dimensional top and bottom viewsillustrating details of the stack of flow battery cells of theembodiment system of FIG. 2A.

FIG. 3E schematically illustrates side cross sectional view of anembodiment of a stack of electrochemical cells in a stack of framesthrough the line A′-A′ in FIG. 3A

FIG. 4A is a schematic illustration of a reservoir and balance of plantportion of a flow battery system according to an embodiment.

FIG. 4B is schematic illustration of a balance of plant plumbingconfiguration of the flow battery system according to an embodiment.

FIG. 4C is a schematic illustration of a balance of plant plumbingconfiguration of a system according to an embodiment in which the bypassexit of the cell stack is ported directly to the concentrated halogenreturn of the electrolyte reservoir.

FIG. 4D is a schematic illustration of a balance of plant plumbingconfiguration of a system according to an embodiment in which where afine filter is placed on the battery cell stack charge inlet.

FIG. 4E is a schematic illustration of a balance of plant plumbingconfiguration of a system according to an embodiment in which the bypassexit and the output of a charge inlet fine filter are both ported to theconcentrated halogen return of the reservoir.

FIG. 4F is a schematic illustration of a balance of plant plumbingconfiguration of a system according to an embodiment in which a finefilter is placed on the bypass exit.

FIG. 4G is a schematic illustration of a balance of plant plumbingconfiguration of a system according to an embodiment in which a finefilter is placed on the joined common and bypass exits of the flowbattery cell stack.

FIG. 5A is a schematic illustration of a balance of plant plumbingconfiguration of a system according to an embodiment in which anadditional pump is added to introduce concentrated halogen to the cellstack instead of direct suction by the main system pump.

FIG. 5B is a schematic illustration of a balance of plant plumbingconfiguration of a system according to an embodiment in whichconcentrated halogen is pumped to the aqueous electrolyte return,creating a mixture of concentrated halogen reactant and aqueouselectrolyte within the reservoir.

FIG. 5C is a schematic illustration of a balance of plant plumbingconfiguration of a system according to an embodiment in which a Venturiinjector is used to move fluid from the concentrated halogen suction tothe aqueous electrolyte return.

FIG. 5D is a schematic illustration of a balance of plant plumbingconfiguration of a system according to an embodiment in which theconcentrated halogen injection point is moved to the discharge inlet.

FIG. 5E is a schematic illustration of a balance of plant plumbingconfiguration of a system according to an embodiment in which a mixingis placed on the joined exits of the battery cell stack.

FIG. 5F is a schematic illustration of a balance of plant plumbingconfiguration of a system according to an embodiment in which an inlineheater element is located on the concentrated halogen injection line.

FIG. 6A illustrates a schematic side cross sectional view of flow pathsin an alternative embodiment electrochemical system.

FIG. 6B illustrates a schematic side cross sectional view of flow pathsin an alternative embodiment electrochemical system.

FIG. 6C illustrates a schematic side cross sectional view of flow pathsin an alternative embodiment electrochemical system.

FIG. 6D illustrates a schematic side cross sectional view of flow pathsin an alternative embodiment electrochemical system.

FIG. 6E illustrates a schematic side cross sectional view of flow pathsin an alternative embodiment electrochemical system.

FIG. 6F illustrates a schematic side cross sectional view of flow pathsin an alternative embodiment electrochemical system.

FIG. 6G illustrates a schematic side cross sectional view of flow pathsin an alternative embodiment electrochemical system.

FIG. 6H illustrates a schematic side cross sectional view of flow pathsin an alternative embodiment electrochemical system.

FIGS. 6I and 6J illustrate schematic side cross sectional views of flowpaths in alternative embodiment electrochemical systems.

DETAILED DESCRIPTION

Embodiments of the present invention are drawn to metal-halogen flowbattery systems and methods of using these systems. The systems includeflow architecture with a single flow circuit. Conventional metal halogenflow batteries maintain electrochemical efficiency by keeping reactantstreams contained in two distinct flow loops by using a separatorbetween the positive and negative electrodes of each flow cell andseparate reservoirs for the electrolyte and the halogen reactant. Theconfigurations below describe systems and methods for reactant handlingthat combine the simplicity and reliability of a single flow loop systemwith reactant separation balance of plant (BOP) components. Preferably,the single flow loop system includes a stack of flow battery cellswithout a separator between the positive and negative electrodes of eachflow cell (i.e., the reaction zone is not partitioned) and a commonreservoir for the electrolyte and the concentrated halogen reactant.

The electrochemical (e.g., flow battery) system can include a vesselcontaining one or more electrochemical cells (e.g., a stack of flowbattery cells) in its inner volume, a metal-halide electrolyte, and aflow circuit configured to deliver the metal-halide electrolyte to theelectrochemical cell(s). The flow circuit may be a closed loop circuitthat is configured to deliver the electrolyte to and from the cell(s).In many embodiments, the loop circuit may be a sealed loop circuit.

Each of the electrochemical cell(s) may comprise a first, fluidpermeable electrode, which may serve as a positive electrode, a second,fluid impermeable electrode, which may serve as a negative electrode,and a reaction zone between the electrodes. The first electrode may be aporous electrode or contain at least one porous element. The firstelectrode may comprise a porous or a permeable carbon, metal or metaloxide electrode. For example, the first electrode may comprise porouscarbon foam, a metal mesh or a porous mixed metal oxide coatedelectrode, such as a porous titanium electrode coated with rutheniumoxide (i.e., ruthenized titanium). In discharge and charge modes, thefirst electrode may serve as a positive electrode at which the halogenmay be reduced into halogen ions. The second electrode may comprise aprimary depositable and oxidizable metal, i.e., a metal that may beoxidized to form cations during the discharge mode. For example, thesecond electrode may comprise a metal that is of the same type as ametal ion in one of the components of the metal halide electrolyte. Forexample, when the metal halide electrolyte comprises zinc halide, suchas zinc chloride or zinc bromide, the second electrode may comprisemetallic zinc. Alternatively, the second electrode may comprise anothermaterial, such as titanium that is plated with zinc.

Preferably, the reaction zone lacks a separator and the electrolytecirculates through the same flow path (e.g., single loop) without aseparation between the electrodes in each cell. In other words, thereaction zone may be such that it does not contain a membrane or aseparator between the positive and negative electrodes of the same cellthat is impermeable to the halogen ions in the electrolyte. Furthermore,the cell may be a hybrid flow battery cell rather than a redox flowbattery cell. Thus, in the hybrid flow battery cell, a metal, such aszinc is plated onto one of the electrodes, the reaction zone lacks anion exchange membrane which allows ions to pass through it (i.e., thereis no ion exchange membrane between the cathode and anode electrodes)and the electrolyte is not separated into a catholyte and anolyte by theion exchange membrane. The electrolyte is stored in one reservoir ratherthan in separate catholyte and anolyte reservoirs.

Preferably, the electrochemical system may be reversible, i.e., capableof working in both charge and discharge operation mode. The reversibleelectrochemical system usually utilizes at least one metal halide in theelectrolyte, such that the metal of the metal halide is sufficientlystrong and stable in its reduced form to be able to form an electrode.The metal halides that can be used in the reversible system include zinchalides, as element zinc is sufficiently stable to be able to form anelectrode. Preferably, the electrolyte is aqueous solution of at leastone metal halide electrolyte compound, such as ZnBr₂ and/or ZnCl₂. Forexample, the solution may be a 15-50% aqueous solution of ZnBr₂ and/orZnCl₂, such as a 25% solution. In certain embodiments, the electrolytemay contain one or more additives, which can enhance the electricalconductivity of the electrolytic solution. For example, when theelectrolyte contains ZnCl₂, such additive can be one or more salts ofsodium or potassium, such as NaCl or KCl. When the electrolyte containsZnBr₂, then the electrolyte may also contain a bromine complexing agent,such as such as a quaternary ammonium bromide (QBr), such asN-ethyl-N-methyl-morpholinium bromide (MEM),N-ethyl-N-methyl-pyrrolidinium bromide (MEP) or Tetra-butyl ammoniumbromide (TBA)).

FIG. 1 illustrates an electrochemical system 100 which includes a stackof flow battery cells in a sealed container 102. The flow battery cellsinside the sealed container 102 are preferably a horizontally positionedcell, which may include a horizontal positive electrode and horizontalnegative electrode separated by a gap. For example, element 103 in FIG.1 represents a vertical stack of horizontally positioned electrochemicalcells (i.e., flow cells) connected electrically in series.

As shown in FIG. 1 a feed (e.g., inlet) conduit (e.g., pipe or manifold115) is configured to deliver the metal-halide electrolyte to thehorizontally positioned cells of the stack 103. A return (e.g., outlet)conduit (e.g., pipe or manifold) 120 is configured to collect productsof an electrochemical reaction from cells of the stack. The return pipeor manifold 120 may be an upward-flowing return pipe or manifold. Thepipe or manifold 120 includes an upward running section 121 and adownward running section 122. The flow of the metal-halide electrolyteand the concentrated halogen reactant leaves the cells of the stack 103upward through the section 121 and then goes downward to the reservoirthrough the section 122. As will be discussed in more detail below, insome embodiments, the feed pipe or manifold and/or the return pipe ormanifold may be a part of a stack assembly for the stack of thehorizontally positioned cells. In some embodiments, the stack 103 may besupported directly by walls of the vessel 102. Yet in some embodiments,the stack 103 may be supported by one or more pipes, pillars or stringsconnected to walls of the vessel 102 and/or reservoir 119.

The flow battery system may include one or more pumps for pumping themetal-halide electrolyte. Such a pump may or may not be located withinthe inner volume of the sealed vessel. For example, FIG. 1 showsdischarge pump 123, which fluidly connects the reservoir 119 and thefeed pipe or manifold 115. The pump 123 is configured to deliver themetal-halide electrolyte through the feed pipe or manifold 115 to thestack of flow battery cell(s) 103. In some embodiments, the flow batterysystem may include an optional additional pump 124. The pump 124 fluidlyconnects the return pipe or manifold 120 to the reservoir 119 and can beused to deliver the metal-halide electrolyte through the return pipe ormanifold to the stack of cell(s) in charge and/or discharge mode.Alternatively, pump 124 may be omitted and the system may comprise asingle flow loop/single pump flow battery system. Any suitable pumps maybe used in the system, such as centripetal and/or centrifugal pumps.

The reservoir 119 may contain a feed line 127 for the concentratedhalogen reactant, which may supply the halogen reactant to the feed pipeor manifold 115 of the system. As used herein, a “concentrated halogenreactant” includes aqueous electrolyte with higher than stoichiometrichalogen content (e.g., higher halogen content than 1:2 zinc to halogenratio for zinc-halide electrolyte), pure liquid halogen (e.g., liquidchlorine and/or bromine) or chemically-complexed halogen, such as abromine-MEP or another bromine-organic molecule complex. A connectionbetween the halogen reactant feed line 127 and the feed pipe manifold115 may occur before, at or after the pump 123. An inlet of the feedline 127 is located in the lower part 126 of the reservoir 119, wherethe complexed bromine reactant may be stored. An outlet of the feed line127 is connected to an inlet of the pump 123. The electrolyte intakefeed line, such as a pipe or conduit 132, is located in the upper part125 of the reservoir 119, where the lighter metal-halide electrolyte(e.g., aqueous zinc bromide) is located.

In some embodiments, the electrochemical system may include acontrolling element, which may be used, for example, for controlling arate of the pump(s). Such a controlling element may be an analogcircuit. FIG. 1 depicts the controlling element as element 128.

Flow Configurations

FIGS. 2B and 2C schematically illustrate respective charge mode anddischarge mode paths for a flow of the metal-halide electrolyte and thehalogen reactant through the horizontally positioned cells of the stack,such as the stack 103 of FIGS. 1 and 2A. The electrolyte flow paths inFIGS. 2A-2C are represented by arrows. The reservoir 119 may contain oneor more internal liquid portions as well as one or more internal gaseousportions. In this embodiment, the reservoir 119 includes two liquidportions 125 and 126, and one gaseous portion 208. Gaseous species, suchas halogen (e.g. Cl₂ or Br₂) and hydrogen gas, are stored in the upperportion 208 (e.g., head space) of the reservoir 119. The reservoir 119may also include internal structures or filters (not shown for clarity).A liquid pump (e.g., centrifugal pump 123) may be used to pump theelectrolyte from upper liquid portion 125 of the reservoir 119 viaconduit 132 which has an inlet in portion 125 of the reservoir. Conduit127 has an inlet in the lower liquid portion 126 of the reservoir 119where the majority of the concentrated halogen reactant is located. Incharge mode, conduit 127 is closed by valve 202 such no concentratedhalogen reactant flows into the stack 103 via conduit 127 during chargemode. In discharge mode, valve 202 is open to allow halogen reactant toflow into the stack 103 via conduit 127.

Each flow battery cell 101 in the stack 103 includes a porous (e.g.,fluid permeable) electrode 23 and a non-porous (e.g., fluid impermeable)electrode 25. As described above, the permeable electrode 23 may be madeof any suitable material, such as a titanium sponge or mesh. Theimpermeable electrode 25 may be made of any suitable material, such astitanium. A layer of metal 25A, such as zinc, is plated on theimpermeable electrode 25 (e.g., on the bottom surface of electrode 25),as shown in FIGS. 2B and 2C. The reaction zone 32 is located between andseparates the impermeable electrode 25/layer of metal 25A and thepermeable electrode 23.

FIG. 2B illustrates the flows through the stack 103 of FIG. 2A duringcharge mode. In the charge mode, aqueous halogen electrolyte is pumpedby the pump 123 from the upper liquid portion 125 of the reservoir 119through conduit 132 into conduit 115. Conduit 115 contains a first flowvalve, such as a proportional three way valve 204. Valve 204 may be acomputer controlled valve. The valve sends a majority (e.g., 51-100%,such as 60-95%, including 70-90%) of the electrolyte into conduit 115A,and a minority (e.g., 0-49%, such as 5-40%, including 10-30%) of theelectrolyte (including no electrolyte) into conduit 115B. Conduit 115Ais fluidly connected to the first stack inlet manifold 1 and conduit115B is fluidly connected to the second stack inlet manifold 2, as willbe described in more detail below.

The first stack inlet manifold 1 provides the major portion of theelectrolyte to the reaction zone 32 of each cell 101, while the secondstack inlet manifold 2 provides a minority of the electrolyte (or noelectrolyte) to the space (e.g., one or more flow channels) 19 betweenthe cells 101 located between the permeable electrode 23 of a first cell101 and an impermeable electrode 25 of an adjacent second cell 101located below the first cell in the stack 103. The electrodes 23, 25 ofadjacent cells may be connected to each other to form a bipolarelectrode assembly 50 as will be described in more detail below. Metal,such as zinc, plates on the bottom of the impermeable electrode 25forming a metal layer 25A in the reaction zone 32. Halogen ions (such aschloride or bromide) in the aqueous electrolyte oxidize to form adiatomic halogen molecule (such as Cl₂, Br₂) on the permeable electrode23.

The majority of the electrolyte flows through the reaction zone 32 andexits into first stack outlet manifold 3. The minority of theelectrolyte (or no electrolyte) flowing in the flow channel(s) 19between the cells 101 exits into the second stack outlet manifold 4.

Manifold 3 provides the electrolyte into conduit 120A while manifold 4provides the electrolyte into conduit 120B. Conduits 120A and 120Bconverge at a second flow valve, such as a proportional three way valve205. Valve 205 may be a computer controlled valve. Valve 205 isconnected to the outlet conduit 120 and controls the electrolyte flowvolume into conduit 120 from conduits 120A and 120B. Conduit 120provides the electrolyte back into the upper liquid portion 25 of thereservoir 119.

Thus, in the charge mode, the metal halide electrolyte is pumped by pump123 from the reservoir 119 through an inlet conduit (e.g., one or moreof flow pathways 132, 115, 115A, 1) to the reaction zone 32 of each flowcell 101 in the stack 103 in one direction (e.g., left to right in FIG.2B). A majority of the metal halide electrolyte enters the reaction zone32 from the inlet conduit (e.g., from manifold 1 portion of the inletconduit) without first flowing through the permeable electrode 23 in theflow cell 101 or through the flow channel 19 located between adjacentflow cell electrodes 23, 25 in the stack 103. The metal halideelectrolyte then flows from the reaction zone 32 of each flow cell inthe stack through an outlet conduit (e.g., one or more of flow pathways3, 120A, 120) to the reservoir 119, such that the majority of the metalhalide electrolyte does not pass through the permeable electrode 23 ineach flow cell 101 before reaching the outlet conduit (e.g., manifold 3portion of the outlet conduit).

FIG. 2C illustrates the flows through the stack 103 of FIG. 2A duringdischarge mode. In discharge mode, valve 202 in conduit 127 is opened,such that the aqueous electrolyte and concentrated halogen reactant(e.g., complexed bromine) are pumped by pump 123 from the respectivemiddle portion 125 and the lower liquid portion 126 of the reservoir 119to respective conduits 132 and 127.

The electrolyte and the concentrated halogen reactant are provided fromrespective regions 125 and 126 of the reservoir 119 via conduits 132 and127. The mixture flows from conduit 115 via valve 204 and conduit 115Aand optionally conduit 115B to respective inlet manifolds 1 and 2. As inthe charge mode, the majority of the electrolyte and concentratedhalogen reactant mixture flows into the inlet manifold 1 and a minorityof the mixture (or no mixture) flows into the inlet manifold 2.

The electrolyte and concentrated halogen reactant (e.g., complexedbromine) mixture enters the reaction zone 32 from manifold 1. In otherwords, the mixture enters the cell reaction zone 32 between theelectrodes 23, 25 from the manifold without first passing through thepermeable electrode 23. Since the complexed bromine part of the mixtureis heavier than the electrolyte, the complexed bromine flows through thepermeable electrode 23 at the bottom of each cell 101. In the dischargemode, complexed bromine passing through the permeable electrode 23 isreduced by electrons, resulting in the formation of bromine ions. At thesame time, the metal layer 25A on the impermeable electrode 25 isoxidized, resulting in metal (e.g., zinc) ions going into solution inthe electrolyte. Bromine ions formed in the discharge step are providedinto the flow channel(s) 19 between the cells 101, and are then providedfrom the flow channel(s) 19 through the second stack outlet manifold 4into conduit 120B. The electrolyte rich in zinc ions is provided fromthe reaction zone 32 through the first stack outlet manifold 3 intoconduit 120A. The bromine ions in conduit 120B and the zinc richelectrolyte in conduit 120A are mixed in valve 205 and then provided viaconduit 120 back to the middle portion 125 of the reservoir.

Thus, in the discharge mode, the mixture of the metal halide electrolyteand the concentrated halogen reactant (e.g., complexed bromine) flowsfrom the reservoir 119 through the inlet conduit (e.g., one or more offlow pathways 132, 115, 115A, 1) to the reaction zone 32 of each flowcell 101 in the stack 103 in the same direction as in the charge mode(e.g., left to right in FIG. 2C). A majority of the mixture enters thereaction zone 32 from the inlet conduit without first flowing throughthe permeable electrode 23 in the flow cells 101 or through the flowchannel 19 located between adjacent flow cell 101 electrodes 23, 25 inthe stack 103. The mixture then flows from the reaction zone 32 of eachflow cell 101 in the stack 103 through the outlet conduit (e.g., one ormore of flow pathways 3, 120A, 120) to the reservoir 119, such that amajority of the mixture passes through the permeable electrode 23 ineach flow cell 101 before reaching the outlet conduit (e.g., themanifold 3 portion of the outlet conduit).

Thus, in charge mode, the majority of the flow is “flow-by” (e.g., themajority of the liquid flows by the permeable electrode through thereaction zone), while in discharge mode, the majority of the flow is“flow-through” (e.g., the majority of the liquid flows through thepermeable electrode from the reaction zone) due to the difference in thereaction kinetics in charge and discharge modes.

In an example of a zinc-bromide flow battery, during charge mode, anelectron is accepted in a reduction process (e.g., Zn²⁺+2e⁻→Zn) at thenegative (e.g., non-porous) electrode of each cell, while electrons aregiven away in an oxidation process (e.g., Br⁻→Br₂+2e⁻) at the positive(e.g., porous) electrode. The process is reversed during the dischargemode. In this example, the electrolyte may be aqueous zinc bromide whilethe concentrated halogen may be liquid bromine, a bromine complex (e.g.,a bromine-MEP complex) or a mixture thereof with the aqueous zincbromide.

Valves 204 and/or 205 may be used control the ratio of liquid flow ratebetween the two inlet paths (e.g., 115A/115B) and/or between the twooutlet paths (e.g., 120A/120B). Thus, the net amount of liquid thatflows through the permeable electrode 23 may be controlled in chargeand/or discharge mode. For example, in charge mode, the valve 205 may beadjusted to provide a higher liquid flow rate through manifold 3 andconduit 120A and a lower liquid flow rate through manifold 4 and conduit120B to favor the “flow-by” flow configuration. In contrast, indischarge mode, the valve 205 may be adjusted to provide a lower liquidflow rate through manifold 3 and conduit 120A and a higher liquid flowrate through manifold 4 and conduit 120B compared to the charge mode tofavor the “flow-through” flow configuration.

In charge mode, the majority of the flow is “flow-by” because this ispreferable for the metal plating reaction and sufficient for the halogenoxidation reaction. For the metal plating reaction, it is important tomaintain an adequate concentration of metal ions (e.g. Zn²⁺) near thesurface of the impermeable electrode 25 onto which the metal layer 25Awill be plated. Insufficient flow speed at the exit end of the platingarea (which might occur in the “flow-through” arrangement used duringdischarge) could lead to metal ion starvation and poor platingmorphology, particularly at high stack open current when the bulkconcentration of metal ions is at its lowest. The halogen oxidationreaction that takes place on the permeable electrode 23 (e.g. bromideions oxidized to bromine) in the charge mode can be adequately suppliedwith reactants in either a “flow-by” or a “flow-through” arrangement.

In contrast, in the discharge mode, the majority of the flow is“flow-through” because this is sufficient for the metal layer 25Ade-plating reaction and preferable for the halogen reduction reaction.The reactant in the metal de-plating reaction (i.e., zinc layer 25A) isalready available along the entire surface of the impermeable electrode25, where it was plated during the charge mode. As a result, both“flow-by” and “flow-through” are adequate to support this reaction. Forthe halogen reduction reaction (e.g. bromine reducing to bromide ions),it is important to supply an adequate concentration of halogen to theactive surface of the permeable electrode 23. The molecular halogen isnot as mobile as its ionic counterpart, particular if a complexing agentis used, so much more surface area and reactant flow rate is needed tosupport the halogen reduction reaction than the halogen oxidationreaction. Flowing through the permeable electrode 23 achieves thisreactant supply requirement.

Thus, charge and discharge inlet flows no longer need to flow onopposite sides of the cell frame and/or in opposite directions. Rather,the same first stack inlet manifold 1 and the same pump 123 may be usedto supply the majority of the flow to the reaction zone 32 during bothcharge and discharge modes. Thus, the majority of the liquid in both thecharge and discharge mode flows in the same direction through thereaction zone in both modes and the majority of the liquid in both thecharge and discharge mode enters the reaction zone 32 directly from theinlet manifold 1 without first flowing through the permeable electrode23 or the flow channel(s) 19 between the cells 101. Thus, manifold 1 maybe referred to as the “main inlet manifold.”

If desirable, the second stack inlet manifold 2 may be used to supply aminority of the flow through the flow channel(s) 19 between the oppositeelectrodes 23, 25 of adjacent flow cells 101 to the bottom side of thepermeable electrode 23 (i.e., the side of electrode 23 facing the flowchannel(s) 19) during charge and/or discharge modes. These charge modeelectrolyte purge flow and/or discharge mode electrolyte-complexedbromine mixture purge flow may be useful to prevent bubbles or densercomplex phase liquid from accumulating beneath the permeable electrode23 in the flow channel(s). Thus, the second stack inlet manifold may bereferred to as the “secondary inlet manifold” or the “purge inletmanifold”. The purge flows flow from the channel(s) 19 to the secondstack outlet manifold 4. Alternatively, the second stack inlet manifold2 and conduit 115B may be omitted to simplify the overall system design.

The flow battery system of FIG. 2A may also include an optionalrecombinator 200 and a gas pump 214. The recombinator is a chambercontaining a catalyst which promotes or catalyzes recombination ofhydrogen and halogen, such as bromine. The gas pump 214 provides halogenand hydrogen gas from the upper portion 208 of the reservoir 119 viaconduit 220 to the recombinator 200. The hydrogen and halogen gasesreact with each other in the recombinator 200 to form a hydrogen-halogencompound. The hydrogen-halogen compound is then returned to the middleportion (e.g., upper liquid portion) 125 of the reservoir 119 from therecombinator 200 via conduits 222 and 120 by the action of the pump 214.

In another embodiment, the pump 214 is replaced with a venturi injector216, as shown in FIG. 2A. Thus, the system preferably contains eitherthe pump 214 or the venturi 216, but in some embodiments the system maycontain both of them. Thus, the venturi is shown with dashed lines. Thehydrogen-halogen compound is drawn from the recombinator 200 intoconduit 222 which merges into the venturi injector. The hydrogen-halogencompound mixes with the electrolyte flow being returned from the stack103 to the reservoir 119 in the venturi injector 206 and the mixture isreturned to the reservoir 119 via the return conduit 120.

FIGS. 3A and 3B illustrate the features of the top and bottom surfaces,respectively, of a cell frame 31 for holding the horizontally positionedflow battery cells illustrated in FIGS. 1 and 2A-2C. The frame 31includes the main inlet manifold 1, the secondary inlet manifold 2 andthe outlet manifolds 3, 4 described above. The manifolds 1-4 arerespective openings through the frame 31 which align with similaropenings in other stacked frames 31 to form the manifolds. Thus, theinlet manifolds 1, 2 are formed by aligned inlet manifold openings inthe stack of cell frames while the outlet manifolds are formed byaligned outlet manifold openings in the stack of cell frames. The framesalso include at least one inlet distribution (e.g., flow) channel and atleast one outlet distribution channel. For example, as shown in FIGS. 3Aand 3B, the upper and lower surfaces of the frame 31 each contain oneinlet distribution channel (e.g., 40 on the upper side and 46 on thelower side) and one outlet distribution channel (e.g., 42 on the upperside and 44 on the lower side). These channels 40-46 comprise grooves inthe respective surface of the frame 31. The distribution (e.g., flow)channels 40, 42, 44, 46 are connected to the active area 41 (e.g.,opening in middle of frame 31 containing the electrodes 23, 25) and to arespective stack inlet or outlet manifold 1, 3, 4 and 2. The inletdistribution channels 40, 46 are configured to introduce the electrolytefrom the respective stack inlet manifold 1, 2 to the reaction zone 32 orthe flow channel(s) 19, and the outlet distribution channels 42, 44 areconfigured to introduce the electrolyte from the reaction zone 32 or theflow channel(s) to the respective outlet manifold 3, 4. Since thedistribution/flow channels 40-46 deliver the electrolyte to and fromeach cell, they may also be referred to as the cell manifolds.

The electrolyte flows from the main inlet manifold 1 through inlet flowchannels 40 and inlet 61 in the frame 31 to the flow cells 101. Asillustrated in FIG. 3A, only the main inlet manifold 1 is fluidlyconnected to the inlet channels 40 on the top of the frame 31. In theembodiment illustrated in FIG. 3A, the charge mode inlet manifold 1connects to two flow channels 40 which successively divide intosubchannels (i.e., flow splitting nodes where each channel is split intotwo subchannels two or more times) to provide a more even and laminarelectrolyte flow to the electrodes 23, 25. After passing across theelectrodes 23, 25, the electrolyte exits the cells from outlet 65 intoexit flow channels 42 on an opposite end or side of the frame 31 fromthe main inlet manifold 1. The electrolyte empties from the exit (i.e.,outlet) flow channels 42 to a first stack outlet manifold 3. Exitchannels 42 may also comprise flow splitting nodes/subchannels as shownin FIG. 3A.

As illustrated in FIG. 3B, on the bottom side of the cell frame 31, thesecond inlet manifold 2 is connected to bottom purge inlet channels 46while the main manifold 1 is fluidly isolated from the purge inletchannels 46. While the secondary inlet manifold 2 is shown as beinglocated closer to the edge of the frame 31 than the main manifold 1 inFIGS. 3A and 3B, the positions of the manifolds 1 and 2 may be reversed.Thus, manifold 1 may be located closer to the frame 31 edge thanmanifold 2, as shown in FIG. 2A or the manifolds 1, 2 may be locatedside by side, as shown in FIG. 4. The second stack outlet manifold 4 isconnected to the electrochemical cells via outlet 66 and bottom exitchannels 44 on the bottom surface of the frame 31.

FIGS. 3C and 3D illustrate the flows through the manifolds in the stackof cell frames 31. The stack of cell frames 31 supports the stack 103 ofcells 101. The stack of cell frames 31 is preferably a vertical stack inwhich adjacent cell frames are separated in the vertical direction.

As shown in FIG. 3C, the majority of the liquid flow in the charge anddischarge mode flows upward through the main inlet manifold 1 in theframes 31. The flow exits the manifold 1 in each frame to two flowchannels 40 which successively divide into subchannels (i.e., flowsplitting nodes where each channel is split into two subchannels two ormore times). The flow then flows from subchannels 40 through outlet 61into the reaction zone 32 of each cell. After passing through thereaction zone between the electrodes 23, 25 of each cell 101, the flowexits the cells from outlet 65 into exit flow channels 42 on an oppositeend or side of the frame 31 from the main inlet manifold 1. The flowempties from the exit flow channels 42 to the first stack outletmanifold 3. As described above, in discharge mode, a portion of the flowpasses through the permeable electrode 23 into the flow channel(s) 19.After passing through the flow channel(s) 19, the flow is providedthrough outlet 66 into exit flow channels 44. The flow empties from theexit flow channels 44 to the second stack outlet manifold 4.

As shown in FIG. 3D, the minority of the liquid flow (e.g., the purgeflow) flows in the charge and discharge mode flows upward through thesecondary inlet manifold 2 in the frames 31. The flow exits the manifold2 in each frame to two flow channels 46 which successively divide intosubchannels (i.e., flow splitting nodes where each channel is split intotwo subchannels two or more times). The flow then flows from subchannels46 through outlet 62 into the flow channel(s) 19 between each cell 101.After passing through the flow channel(s) 19, the flow is providedthrough outlet 66 into exit flow channels 44. The flow empties from theexit flow channels 44 to the second stack outlet manifold 4.

As described above with respect to FIGS. 2B and 2C, in charge mode, thepurge flow passes through outlets 66 channels 44 to manifold 4. Indischarge mode, the majority of the flow passes through the permeableelectrode 23 into channel(s) 19 and then through outlet 66 into exitchannels 44 and then into manifold 4. Thus, the purge flow may beomitted in discharge mode by adjusting valve 204 to close line 115B.

FIG. 3E illustrates a cross section of an embodiment of a stack ofelectrochemical cells in a stack of frames through the line A′-A′ inFIG. 3A. The cross section A′-A′ is transverse to the flow ofelectrolyte in the electrochemical cell from inlet manifolds 1, 2 tooutlet manifolds 3, 4. In this embodiment, the frame 31 includes ledges33 on which the non-permeable (negative) metal electrode 25 is seated.Additionally, the non-permeable electrode 25 of a first electrochemicalcell 101 a is spaced apart from and connected to the permeable(positive) electrode 23 of an adjacent, overlying electrochemical cell101 b by one or more electrically conductive spacers 18, such as metalor carbon spacers. An electrolyte flow channel 19 is thereby formedbetween the non-permeable electrode 25 of the first electrochemical cell101 a and the overlying permeable electrode 23 of an adjacentelectrochemical cell 101 b. Further, if plural conductive spacers 18 areused, then the spacers divide the electrolyte flow path 18 into a seriesof flow channels 19.

In an embodiment, the electrodes 23, 25 of adjacent electrochemicalcells 101 are provided as an assembly 50. In this embodiment, thenon-permeable electrode 25 of a first electrochemical cell 101 a, theconductive spacers 18 separated by channels 19 and the porous electrode23 of the adjacent electrochemical cell 101 b are assembled as a singleunit. The individual components may be glued, bolted, clamped, brazed,soldered or otherwise joined together. The fabrication of an electrodeassembly 50 simplifies and speeds the assembly of stacked flow celldevice. Each electrode assembly is placed into a respective frame 31,such that one electrode (e.g., the larger non-permeable electrode 25) issupported by the ledges 33 in the frame 31, and the other electrode(e.g., the smaller non-permeable electrode 23) is supported in the space41 between the ledges 33 by the spacers 18 from the underlyingnon-permeable electrode 25. Of course the order of the electrodes may bereversed and the porous electrode may be supported by the ledges 33.Other electrode attachment configurations, such as bolting or clampingto the frame, may be used. The frames 31 with the electrodes 23, 25 arestacked upon each other to form the stack 103 of cells. As each frame isstacked, a new cell 101 is created with a reaction zone 32 in betweenthe bottom electrode 23 and a top electrode 25 of each cell. As seen inFIGS. 2A-2C, the electrodes 23, 25 of the same cell (e.g., 101 a) areseparated by the reaction zone 32 and do not physically or electricallycontact each other and comprise a portion of separate electrodeassemblies.

As described above, the flow battery system illustrated in FIGS. 1-3Econtains two types of flow manifolds: stack manifolds 1, 2, 3 and 4which are common flow paths that feed individual cell flow paths, andcell manifolds 40, 42, 44 and 46 which are flow paths that distributeflow from (or to) the stack manifold to (or from) the entire width ofthe active area in an individual flow cell. Preferably, as describedabove and illustrated in FIGS. 3A and 3B, the stack manifolds (e.g.,aligned holes in a stack of cell frames 31) and cell manifolds (e.g.,grooves in the cell frames 31) are formed directly into the cell frames31 that house and align the electrodes in a stack assembly. Thiseliminates the cost and complexity associated with external manifoldplumbing (e.g., large tube feeding multiple small tubes) found in priorart flow batteries. Additionally, the integration of the stack and cellmanifolds into the cell frame ensures that the stack and cell manifoldsare fully contained within the primary stack sealing envelope shown inFIG. 12. As a result, the flow channel seals are not integral to theseal between the stack and the vessel 102, reducing the overall leakrisk.

The flow battery system also includes and electrolyte reservoir 200illustrated in FIG. 4A. An embodiment of the electrolyte reservoir 200includes a vessel 201 (e.g., tank or other suitable fluid container)with two outlets (also referred to as stack feed lines) 202, 204 and twoinlets (also referred to as reservoir return lines from the stack) 206,208. The outlets 102, 104 and inlets 106, 108 may each be an openingand/or a conduit (e.g., pipe or manifold line) leading from thereservoir 200 to stack 100 and from the stack 100 back to reservoir,respectively. There are separate aqueous electrolyte and concentratedhalogen suction feed lines 202, 204 to allow access to regions withinthe reservoir of varying fluidic composition (e.g., upper lighterelectrolyte and lower heavier concentrated halogen regions). There areseparate aqueous electrolyte and concentrated halogen return lines 206,208 to allow for separate return streams to be provided to differentportions within the reservoir.

An embodiment of a metal-halogen flow battery system 300 illustrating anembodiment of a balance of plant (BOP) plumbing configuration is shownin FIG. 4B. In charge mode, aqueous electrolyte is pumped from reservoir200 by pump 304 through line 202. An actuated valve 302A can distributethe electrolyte flow to the charge inlet 102 and/or discharge inlet 104of the cell stack 100 in charge mode. Preferably, the majority or all ofthe electrolyte is provided to the stack through inlet 102 through valve302A. In charge mode, valve 302B in line 204 is closed so that theconcentrated halogen is not provided from the reservoir 200 into stack100 through line 204. The common and bypass exits 106, 108 of the cellstack 100 are joined to a common return line 306 which is connected tothe aqueous electrolyte return 206 of the reservoir 200. Preferably, theexits 106, 108 are joined to the common return line 306 outside thestack 100. For example, exits 106, 108 may be conduits which separatelyextend outside the stack enclosure or frame before merging into a commonconduit 306. The concentrated halogen return line 208 of the reservoir200 is omitted in this embodiment. Thus, all electrolyte flows from thestack through line 306 to the reservoir.

In discharge mode, the actuated valve 302B on the concentrated halogenfeed line (i.e., suction pathway) 204 is opened which allows the mainsystem pump 304 to provide simultaneous suction of aqueous electrolytefrom the upper part of the vessel 201 via line 202 a and of theconcentrated halogen reactant from the lower part of the vessel 201 vialine 204. This high halogen-content fluid is provided to the cell stack100 through valve 302A and inlet 104. The electrolyte outlet flow fromthe stack 100 into the reservoir 200 in discharge mode is the same as inthe charge mode in this embodiment.

FIGS. 4C-4G illustrate embodiments of metal-halogen flow battery systems400A-400E with different BOP configuration features that may be usedsingly or in any combination to reduce the concentrated halogen contentof circulating electrolyte. FIG. 4C shows a BOP in which the bypass exit108 of the cell stack 100 is ported directly to the concentrated halogenreturn line 208 of the electrolyte reservoir 200, while the common exit106 is connected to the aqueous electrolyte return 206.

FIG. 4D shows a configuration where a fine filter 406, such as ahydrocyclone, coalescer, or other device that separates suspensionsbased on physiochemical differences, is placed on the battery cell stackcharge mode inlet 102. The filter 406 reduces the concentrated halogencontent of the electrolyte entering the charge mode inlet 102 andprovides a concentrated halogen stream that bypasses the battery cellstack 100 via a bypass channel 308 and is ported directly to theconcentrated halogen return 208 of the reservoir 200.

FIG. 4E illustrates an embodiment that includes a combination of thefeatures of the embodiments illustrated in FIGS. 4C and 4D. In thisembodiment, the bypass exit 108 and the output of a charge inlet finefilter 406 are both ported to the concentrated halogen return 208 of thereservoir 200.

FIG. 4F illustrates an embodiment in which a fine filter 406 is placedon the bypass exit 108. The filter 406 provides a concentrated halogenstream that can be ported to the concentrated halogen return 208 of thereservoir 200 and an aqueous electrolyte stream that may be ported tothe aqueous electrolyte return 206 of the reservoir 200.

FIG. 4G illustrates an embodiment in which a fine filter 406 is placedon the joined common and bypass exits 306 of the cell stack 100 of FIG.4B. The filter 406 provides a concentrated halogen stream that can beported to the concentrated halogen return 208 of the reservoir 200 andan aqueous electrolyte stream that may be ported to the aqueouselectrolyte return 206 of the reservoir 200.

FIGS. 5A-5F illustrate embodiments of metal-halogen flow battery system500A-500F with different BOP configuration features that allow for theintroduction and remixing of concentrated halogen reactant with theaqueous electrolyte during battery discharge mode. As illustrated, theseembodiments are based on FIG. 4B, but they can be used in concert withany of the BOP configurations features shown in FIGS. 4C-4G. FIG. 5Aprovides a metal-halogen flow battery system 500A similar to FIG. 3, butuses a second pump 504 to introduce concentrated halogen to stack 100 indischarge mode instead of direct suction by the main system pump 304.Pump 504 is located on the concentrated halogen feed line 204. Thus, inthis embodiment, valve 302B may be omitted because the pump 504 performsthe valving function by turning on in discharge mode and off in chargemode.

In the metal-halogen flow battery system 500B illustrated in FIG. 5B,the pump 504 is also located on line 204. However, in this embodiment,line 204 connects to the common return line 306 instead of to the mainpump 304 and valve 302A. The concentrated halogen is pumped to theaqueous electrolyte return 206, creating a mixture of concentratedhalogen reactant and aqueous electrolyte within the reservoir 200. Thishalogen-enriched fluid may be used in discharge mode when the main pump304 suctions the fluid from the upper part of the vessel 201 into line202.

The metal-halogen flow battery system 500C illustrated in FIG. 5C issimilar to the metal-halogen flow battery system 500B illustrated inFIG. 5B, but it uses a Venturi injector 502 to move fluid from theconcentrated halogen suction outlet 204 to the aqueous electrolytereturn 206. An actuated valve 302B may be placed on the concentratedhalogen suction outlet 204 to allow concentrated halogen flow onlyduring discharge mode but not in charge.

The metal-halogen flow battery system 500D illustrated in FIG. 5D movesthe concentrated halogen injection point to the discharge mode inlet104. The concentrated halogen may be either pumped or suctioned using aVenturi injector in discharge mode through the open actuated valve 302B.A physical mixing device 506, such as a nozzle, static mixer, ultrasonicemulsifier, etc., may be located between the concentrated halogeninjection point and the battery cell stack 100 to disperse the injectedfluid into the bulk electrolyte flow in inlet 104.

FIG. 5E shows an embodiment of a metal-halogen flow battery system 500Ewith a mixing device 506, such as a nozzle, static mixer, ultrasonicemulsifier, etc., placed on the joined exit 306 of the battery cellstack. This mixer helps to homogenize the suspension of electrolyte andconcentrated halogen reactant leaving the battery cell stack 100 viaexits 106, 108, creating a better discharge fluid in the reservoir 200.

FIG. 5F illustrates an embodiment of a metal-halogen flow battery system500F with an inline heater element 508 on the concentrated halogen feedline 204. Heating the concentrated halogen flow stream alters thephysical and chemical properties of the concentrated halogen flow streamand may facilitate mixing. Heating the concentrated halogen flow streammay also make more halogen available for the discharge reaction in thebattery cell stack 100. An inline heater 508 could also be added to anyof the embodiments illustrated in FIGS. 5A-5E.

FIGS. 6A-6D schematically illustrate alternative flow paths for a flowof the metal-halide electrolyte and the halogen reactant through thehorizontally positioned cells of a stack, such as the stack 103 of FIGS.1 and 2A. The electrolyte flow paths in FIGS. 6A-6D are represented byarrows. For brevity, and in order to allow comparison with theelectrolyte flow paths previously discussed, components illustrated inand discussed above with respect to FIGS. 2A-2C, are identified in FIGS.6A-6D with the same reference numerals.

In an alternative embodiment shown in FIG. 6A, manifold 3 provides theelectrolyte into conduit 120A while manifold 4 provides the electrolyteinto conduit 120B. Conduits 120A and 120B separately provide outlet(i.e., exit) flow streams to the reservoir 119, and have separate flowcontrol valves 205 a and 205 b, respectively (instead of the three wayvalve 205 in FIG. 2A). In this manner, the tendency of the complexhalogen to settle out and collect in the discharge exit path in conduit120A may be avoided. That is, preserving the concentrated stream ofcomplex halogen and returning it to a separate location may enableeasier storage and management of the complex phase. Also, to control theflow ratios of the main inlet line and purge inlet line, conduits 115Aand 115B may be configured with control flow valves 117 a and 117 b,respectively. If the majority of the flow enters the main inlet conduit115A in all operational modes, then flow control valve 117 a may beeliminated.

In another alternative embodiment, shown in FIG. 6B, conduits 120A and120B separately provide exit flow streams to the reservoir 119, similarto the embodiment discussed above with respect to FIG. 6A. In thisembodiment, however, conduits 120A and 120B may be configured withcalibrated pipe restrictions 602 a, 602 b and on/off valves 604 a, 604b, in order to control the flow ratios of the exit flow streams. Also,to control the flow ratios of the main inlet line and purge inlet line,conduits 115A and 115B may be configured with calibrated piperestrictions 606 a, 606 b and on/off valves 608 a, 608 b. The piperestrictions comprise a narrow pipe or orifice that has a smaller widthor diameter than conduits 120A, 120B. If the majority of the flow entersthe main inlet conduit 115A in all operational modes, then flow controlvalves 117 a, 117 b and restriction 606 a may be eliminated to leaveonly the restriction 606 b.

In another alternative embodiment, shown in FIG. 6C, the output conduits120A, 120B may be fluidly connected to a majority outlet flow conduit120 c and a minority outlet flow conduit 120 d. The majority of theoutlet (i.e., exit) flow always flows through conduit 120 c in bothcharge and discharge modes, while the minority of the outlet flow flowsthrough conduit 120 d in both charge and discharge modes. A calibratedpipe restriction 602 is located in conduit 120 d but not in conduit 120c. On/off valves 610 a, 610 b, 610 c and 610 d may be used to steer theoutlet (i.e., exit) flows from manifolds 3 and 4 through variousconduits 120 a-120 d into the reservoir 119.

In this configuration, the exit flow return locations are differentiatedby flow rate, rather than the flow path from which they originated. Forexample, in charge mode, the majority of the outlet flow flows fromreaction zone 32, through manifold 4, into conduit 120B, while theminority of the outlet flow or no outlet flow flows from region 19through manifold 3 into conduit 120A. In charge mode, on/off valves 610a and 610 c are open and valves 610 b and 610 d are closed. This valveconfiguration forces the minority of the outlet flow to travel fromregion 19 through manifold 3, conduit 120A, valve 610 a and through thecalibrated pipe restriction 602 in conduit 120 d to the reservoir, whilethe majority of the outlet flow travels from reaction zone 32 throughmanifold 4, conduit 120B, valve 610 c and conduit 120 c into thereservoir.

In the discharge mode, the valve configuration is reversed, on/offvalves 610 a and 610 c are closed and valves 610 b and 610 d are open.This valve configuration forces the minority of the outlet flow totravel from the reaction zone 32 through manifold 4, conduit 120B, valve610 d, bypass conduit 120 f and through the calibrated pipe restriction602 in conduit 120 d to the reservoir, while the majority of the outletflow travels from region 19 through manifold 3, conduit 120A, valve 610b, bypass conduit 120 e and conduit 120 c into the reservoir. Thus, inboth modes, the majority of the flow bypasses the restriction 602 whilethe minority of the flow flows through the restriction.

While four on/off valves are illustrated in FIG. 6C, multi-way valve(s)may be used instead to direct the flows between conduits 120A, 120B andconduits 102C and 120D. This arrangement of FIG. 6C may be preferable ifthere is a device downstream of the stack that operates best underspecific flow conditions.

In another alternative embodiment, shown in FIG. 6D, the main inlet isprovided by conduit 115, through which electrolyte may flow from thereservoir 119 to the manifold 1. In contrast to other embodimentsdiscussed herein, no purge inlet or inlet flow control valve is providedin this embodiment configuration. Thus, conduit 115B and manifold 2 areomitted in this embodiment and there is only one common inlet conduit115 and inlet manifold 1 for both charge and discharge modes. Conduits120A and 120B may be configured with calibrated pipe restrictions 602 a,602 b and on/off valves 604 a, 604 b, in order to control the flowratios of the exit flow streams, similar to the embodiment describedabove with respect to FIG. 6B. Valve 604 a is closed and valve 604 b isopen in charge mode. In contrast, valve 604 a is open and valve 604 b isclosed in discharge mode. Thus, fixed restriction should be sufficientto control the amount of flow going into each outlet path, in whichallows the use of pair of cheaper on/off valves rather than a morecostly flow control valve.

FIGS. 6E-6H schematically illustrate alternative embodimentscorresponding to the embodiments shown in FIGS. 6A-6D, respectively. Ineach of FIGS. 6E-6H, the upper electrode in each cell is a permeableelectrode 23, and the lower electrode in each cell is an impermeableelectrode 25, whereas FIGS. 6A-6D show the opposite electrodeconfiguration. In contrast to the Zn plating in FIGS. 6A-6D, whichoccurs on the bottom face of impermeable electrode 25 against gravity,in FIGS. 6E-6H, the plating of Zn occurs on the top face of impermeableelectrode 25. All other features in FIGS. 6E-6H are similar to FIGS.6A-6D. Of course the alternative electrode configuration described abovefor FIGS. 6E-6H may also be used in the system shown in FIG. 6A.

Referring back to the alternative flow paths for a flow of metal-halideelectrolyte and the halogen reactant through the horizontally positionedcells of a stack, FIG. 6I schematically illustrates another alternativeembodiment. Similar to FIG. 6D, the main inlet is provided by conduit115, through which electrolyte may flow from the reservoir 119 to themanifold 1. Thus, in this embodiment of FIG. 6I there is one commoninlet conduit 115 and inlet manifold 1 for both charge and dischargemodes. In contrast to the embodiment illustrated in FIG. 6D, no outleton/off valves are provided for conduits 120A and 120B in thisembodiment. Conduit 120B may be configured with a calibrated piperestriction 602 b in order to control the flow ratio of the flow streamsbetween conduits 120A and 120B. Preferably conduit 120A lacks therestriction. By placing the calibrated flow restriction 602 b in conduit120B, fluid dynamics may force a majority of fluid flow (e.g., 80%) fromreaction zone 32 through the porous electrode 23 and region 19 tomanifold 3 and conduit 120A in both the charge and discharge modes. Atthe same time, a minority of the fluid flow (e.g., 20%) may exit fromreaction zone 32 through manifold 4 and conduit 120B without flowingthrough the porous electrode 23. The fixed restriction should besufficient to control the amount of flow into each outlet path, thusallowing for a simpler and more reliable system by having fewer valvesand having cell geometry optimized for one flow condition.

FIG. 6J schematically illustrates another alternative embodiment.Similar to the embodiment illustrated in FIG. 6I, the main inlet isprovided by conduit 115 through which electrolyte may flow from thereservoir 119 to the manifold 1. However, in contrast to the embodimentillustrated in FIG. 6I and similar to the embodiment illustrated in FIG.2A, the reservoir 119 contains two feed lines 127, 132. An inlet of thefeed line 127 is located in the lower part 126 of the reservoir 119,where the complexed bromine reactant (i.e., the concentrated halogenreactant) may be stored. In contrast to the embodiment illustrated inFIG. 2A, an outlet of the feed line 127 is connected to an inlet of asecond pump and/or valve 623. An inlet of the second electrolyte intakefeed line (e.g. pipe or conduit 132) is located in the upper part 125 ofthe reservoir 119, where the lighter metal-halide electrolyte (e.g.,aqueous zinc bromide) is located. In the embodiment illustrated in FIG.6J, the pipe or conduit 132 exits through a bottom wall of the reservoir119. However, the pipe or conduit 132 may be configured to exit througha side wall of the reservoir 119 as illustrated in FIG. 2A. Further,unlike the pipe or conduit 132 illustrated in FIG. 2A, the outlet of thepipe or conduit 132 of the present embodiment is not connected to avalve 202. Rather, the outlet of the feed line 132 is connected to aninlet of the pump 123. In charge mode, conduit 127 is closed by shuttingoff the valve and/or pump 623 (e.g., turning off the pump) such noconcentrated halogen reactant flows into the stack 103 via conduit 127during charge mode. In discharge mode, valve and/or pump 623 is open(e.g., the pump is turned on) to allow the concentrated halogen reactantto flow into the stack 103 via conduit 127. Preferably, conduit 127 isconnected to conduit 132 upstream of the pump 123 such that the pump 123can draw both the concentrated halogen reactant from conduit 127 and themetal-halide electrolyte from conduit 132 into conduit 115 to beprovided into the stack 103.

Additionally, similar to the embodiments illustrated in FIGS. 6B, 6H and6I, conduits 120A and 120B separately provide exit flow streams from thestack 103 to the reservoir 119. In this embodiment, however, conduits120A and 120B may be configured with variable flow control valves 604av, 604 bv, in order to control the flow ratios of the exit flow streamsrather than calibrated pipe restrictions 602 a, 602 b and on/off valves604 a, 604 b.

Thus, in this embodiment of FIG. 6J there is one common inlet conduit115 and inlet manifold 1 for both charge and discharge modes, similar tothat shown in FIG. 6I. The variable valves 604 av and/or 604 bv may beconfigured to force the majority of fluid flow (e.g., >50%, such as60-90%, for example 80%) from reaction zone 32 through the porouselectrode 23 and region 19 to manifold 3 and conduit 120A in both thecharge and discharge modes, while forcing a minority of the fluid flow(e.g., <50%, such as 10-40%, for example 20%) to exit from reaction zone32 through manifold 4 and conduit 120B without flowing through theporous electrode 23. Alternatively, the variable valves 604 av and/or604 bv may be configured to force the minority of fluid flow (e.g. <50%,such as 10-40%, for example 20%) from reaction zone 32 through theporous electrode 23 and region 19 to manifold 3 and conduit 120A in boththe charge and discharge modes, while forcing a majority of the fluidflow (e.g., >50%, such as 60-90%, for example 80%) to exit from reactionzone 32 through manifold 4 and conduit 120B without flowing through theporous electrode 23.

Although the foregoing refers to particular preferred embodiments, itwill be understood that the invention is not so limited. It will occurto those of ordinary skill in the art that various modifications may bemade to the disclosed embodiments and that such modifications areintended to be within the scope of the invention. All of thepublications, patent applications and patents cited herein areincorporated herein by reference in their entirety.

What is claimed is:
 1. A method of operating a flow battery comprising astack of flow cells where each flow cell in the stack comprises a fluidpermeable electrode, a fluid impermeable electrode, and a reaction zonebetween the permeable and impermeable electrodes, the method comprising:(a) in charge mode, plating a metal layer on the impermeable electrodeof each cell in the reaction zone by: (i) flowing a metal halideelectrolyte from a reservoir through an inlet conduit to the reactionzone of each flow cell in the stack in a first direction, such that amajority of the metal halide electrolyte enters the reaction zone fromthe inlet conduit without first flowing through the permeable electrodein the flow cell or through a flow channel located between adjacent flowcell electrodes in the stack; and (ii) flowing the metal halideelectrolyte from the reaction zone of each flow cell in the stackthrough a first outlet conduit to the reservoir, such that the majorityof the metal halide electrolyte passes through the permeable electrodein each flow cell before reaching the first outlet conduit; and (b) indischarge mode, de-plating the metal layer on the impermeable electrodeof each cell in the reaction zone by: (i) flowing a mixture of the metalhalide electrolyte and a concentrated halogen reactant from thereservoir through the inlet conduit to the reaction zone of each flowcell in the stack in the first direction, such that a majority of themixture enters the reaction zone from the inlet conduit without firstflowing through the permeable electrode in the flow cells or through theflow channel located between adjacent flow cell electrodes in the stack;and (ii) flowing the mixture from the reaction zone of each flow cell inthe stack through the first outlet conduit to the reservoir, such that amajority of the mixture passes through the permeable electrode in eachflow cell before reaching the first outlet conduit.
 2. The method ofclaim 1, wherein: the stack of flow cells comprises a vertical stack ofhorizontally positioned flow cells; the stack of flow cells is supportedby a stack of cell frames; a portion of the inlet conduit comprises aninlet manifold; a portion of the first outlet conduit comprises a firstoutlet manifold; a portion of the second outlet conduit comprises asecond outlet manifold; each cell frame in the stack of cell framescomprises an inlet manifold opening and a first and a second outletmanifold openings; the first inlet manifold is formed by aligned inletmanifold openings in the stack of cell frames; and the first and thesecond outlet manifolds are formed by respective aligned first andsecond outlet manifold openings in the stack of cell frames.
 3. Themethod of claim 1, wherein: the metal halide electrolyte comprises zincbromide; the concentrated halogen reactant comprises complexed bromine;the metal layer comprises zinc; the permeable electrode comprises porousruthenized titanium; and the non-permeable electrode comprises titaniumthat is coated with the zinc metal layer during the charge mode.
 4. Themethod of claim 1, further comprising: in charge mode, flowing the metalhalide electrolyte from the reaction zone of each flow cell in the stackthrough a second outlet conduit such that a minority of the metal halidedoes not pass through the permeable electrode in each flow cell beforereaching the second outlet conduit; and in discharge mode, flowing themixture from the reaction zone of each flow cell in the stack throughthe second outlet conduit to the reservoir, such that a minority of themixture does not pass through the permeable electrode in each flow cellbefore reaching the second outlet conduit.
 5. The method of claim 4,further comprising controlling a flow rate of outlet flow streams in thefirst and second outlet conduits by a flow restrictor located in thesecond outlet conduit and providing separate outlet flow streams intothe reservoir.
 6. The method of claim 4, wherein: the majority flow forthe charge and discharge mode is greater than 50%; and flowing a mixtureof the metal halide electrolyte and a concentrated halogen reactant fromthe reservoir comprises flowing the metal halide electrolyte through afirst feed line fluidly connected to the reservoir and flowing theconcentrated halogen reactant through a second feed line fluidlyconnected to the reservoir.
 7. The method of claim 6, wherein the firstfeed line comprises an intake fluidly connected to an upper portion ofthe reservoir and the second feed line comprises an intake fluidlyconnected to a lower portion of the reservoir.
 8. The method of claim 7,further comprising mixing the metal halide electrolyte and theconcentrated halogen reactant prior to providing the mixture to a firstpump in a single flow loop.
 9. The method of claim 8, further comprisingpumping the concentrated halogen reactant with a second pump prior tomixing.
 10. The method of claim 1, wherein in the charge mode noconcentrated halogen reactant flows into the stack via the second feedline and in the discharge mode the concentrated halogen reactant flowsinto the stack via the second feed line.
 11. The method of claim 1,further comprising controlling an amount of flow through each flow cellby adjusting at least one of a first variable valve located in the firstoutlet conduit and a second variable valve located in the second outletconduit.